PF has several ways to get a shield bonus without actually using a shield.
The most common is to fight defensively while using a blocking weapon (the nonmagic weapon ability). Others include the improvised defence trait, the orc weapon expertise feat, the meteor hammer in fortress mode and so on.
As non of those options relies on a shield you can't add shield enhancements to them (at least that's what I guess). But are there other ways?
For the meteor hammer I guess you could enhance one side with defending and apply that in fortress mode because you still seem to use (wield) both ends). But that seems expensive. And I'm more interested in the improvised defence trait.
It has to be an improvised weapon and as such can't be enchanted (or can it? If I choose the combat scabbard for example which is on the weapon list). But even if that would not help unless I'm using it to attack, at which time I loose the shield bonus.